Notes

The Great Wall of China, a huge stone structure stretching from the Yellow Sea to the Asian deserts, was built over a period of approximately
1,800 years. The wall is 25 feet high and 12 feet thick, and runs nearly 1,500 miles across northern China. The purpose of the Great Wall was
to make it difficult for raiders to escape with their booty, and thereby discourage invasion. Despite its overwhelming size, the wall was not intended
to keep invaders out, since manning such a defensive structure would have been prohibitively expensive.